Generally, in a light scanning device having a galvanometer mirror as light deflection means, it is necessary to detect a light beam outside an electrostatic latent image formation area on a photosensitive body, for example, near an end portion of the light scanning surface on the photosensitive body in order to control an oscillation angle of the mirror or control a write start position of an electrostatic latent image (picture image) on the photosensitive body. Therefore, JP-A-H9-230278 describes a light scanning device including a detection sensor for detecting the light beam at a position corresponding to each of a scan start position and a scan end position on the photosensitive drum.
In the above-described light scanning device, each detection sensor is disposed such that an optical path length from each detection sensor to the galvanometer mirror is equal to each other. A light beam is reflected from a reflection mirror of the galvanometer mirror to pass through a scanning lens, and then led to the detection sensor. In this path, the light beam is converted to allow a constant speed scanning on the photosensitive drum (light scanning surface) when passing through the scanning lens.
If the optical path length from each detection sensor to the galvanometer mirror is equal to each other, the speed (time) at which the light beam passes through the detection position of each detection sensor is equal, so that the signal obtained by each detection sensor, which usually can not determine the scanning direction but has the same electric characteristics, is equal. In this case, the scanning direction of the light beam can not be determined. This is disadvantage is remarkable especially in the case where the signal is inputted from a single input unit into a control system for controlling light deflection means, that is, only one detection sensor is provided, or where only one input unit of the signal is provided for a plurality of detection sensors.